The invention relates to a metallic object with a thin polyphase oxide coating and a process for the manufacture thereof. Objects with such an oxide coating exhibit, in addition to advantageous chemical and physical properties, high biocompatibility and can be used for a range of purposes due to their properties.
Known polyphase oxide coatings on metallic materials are produced by methods that utilize interdiffusion processes at high temperatures, or try to achieve a polyphase coating by deposition techniques with alternating coatings (flame spraying, PVD). Those coatings can also be produced by the sol-gel technology through a treatment at high temperatures. Common to all these methods is the fact that they are performed, at least partly, at process conditions that, particularly due to high temperatures, make the incorporation of organic phases impossible and, for inorganic phases, predominantly lead to the incorporation of waterfree high-temperature modifications.
A method for the production of modified, if necessary, oxide ceramic coatings on metals forming a barrier layer (valve metals; Ti, Al, Zr, etc.) is presented in EP 0545230. These oxide ceramic coatings are produced by plasma-chemical anodic oxidation in a chloride-free electrolyte bath having a pH-value of 2 to 8 by reaction at temperatures of xe2x88x9230xc2x0 C. to +15xc2x0 C. In this process no alloy is formed between the metal oxide phase and other inorganic phases. Due to the local plasma-chemical conditions at the place of oxide formation any organic substances are completely destroyed.
From DE-OS 36 27 249 a method is known by which conversion coatings on titanium surfaces are produced which consist of high-molecular organic compounds and tensides. These coatings are characterized by a very good adhesive strength, but are not realized through alloying of metal oxide with another phase. Furthermore, coating is executed at temperatures of 40-80xc2x0 C. which excludes use of proteins.
From EP 0232791 and EP 0237053 methods are known in which a resorbable calcium phosphate ceramic, which is contained in oxides, is applied to titanium by anodic oxidation in aqueous electrolytes during spark discharge. The coatings thereby produced, however, do not consist of hydroxyapatite or fluoroapatite but of oxides and easily resorbable calcium phosphates. With the complete resorption of the calcium phosphate phases also the bioactive character of the implant is lost. Since also in this method the oxide coating formation occurs during spark discharge, any organic substances are completely distroyed.
In CA 2,073,781 A1 a method is presented in which an oxide coating is formed by anodic oxidation of the metals (titanium) or alloys (Ti- and Co-base alloys) used and, by subsequent cathodic polarization, calcium phosphate phases with different crystal modifications are deposited on the anodically formed oxide coating. The thereby produced coatings are to be treated with biologically active substances, such as collagens, BMP (bone morphogenetic proteins) or antibiotic substances. By using this method, the organic phases cannot be incorporated into the electrochemically formed surface coatings.
WO 92/13984 describes a method for deposition of bioactive coatings on conductive substrates. An electrolytic cell contains an inert anode and an electrolyte solution, which consists of an aqueous solution of ions of the ceramic and exhibits a pH-value of less than 8. The activated conductive substrate is immersed into the electrolyte solution and the potential between anode and conductive substrate set such that a ceramic coating is deposited on the conductive substrate by an increase of the pH-value at the interface between electrolyte solution and conductive substrate. It is a disadvantage of the solution that the coating is deposited only on the surface of the substrate so that, firstly, no load-resistant connection to it can be formed and, secondly, the coating is biologically completely resorbable.
It is an objective of the invention to create metallic objects with an improved surface coating by the production of thin polyphase oxide coatings under process conditions that allow the incorporation of organic and/or inorganic phases.
According to the invention the problem is solved by a metallic object with a thin polyphase oxide coating, whereby the oxide coating consists of a metallic oxide phase and at least one other organic and/or inorganic component. The metallic object consists of a valve metal, such as aluminum, titanium, tantalum, circonium, niobium, or its alloy, inclusive of intermetallic phases. Oxide coatings formed on these metals or alloys, respectively, show ionic conduction, at least when anodically polarized, and thereby, through anodic polarization, allow to vary the thickness of the oxide coatings within wide limits.
The distribution of the oxide coating growth to the phase boundaries metallic substrate material/oxide and oxide/environment can hereby be controlled through the selected electrochemical conditions. In this way two-layer oxide coatings can be produced, the outer layer of which may contain inorganic and/or organic phases, whereby the total thickness of the oxide coating as well as the distribution of the total thickness relative to the two coating componets can be controlled by selection of the electrochemical parameters potential, current and potential change rate. This makes it possible, depending on the particle size of the phases to be incorporated into the oxide coating, either to completely incorporate them or to adjust a defined degree of incorporation.
The organic component preferably consists of polymer materials, such as collagen, S-layer, polycarbonate and fullerenes, and/or biomolecules, and/or oligomers.
The inorganic component is preferably formed of inorganic fiber structures or calcium phosphate phases. It can be incorporated into the oxide phase of the metallic material, either alone or in connection with the organic component, or as a composite with the organic component.
The organic and/or inorganic component is inventively incorporated into the metallic oxide phase such that the polyphase oxide coating compares with an alloy. The organic component can extend beyond the polyphase oxide coating.
According to the invention, a thin polyphase oxide coating is produced on a metallic substrate material in such a way that first the metallic substrate material is brought into contact with the organic and/or inorganic phases to be integrated into the oxide coating such that these phases are present at, or in direct vicinity of the surface of the substrate.
The contact with the phases to be integrated into the oxide coating can be realized through adsorption, sedimentation, application, deposition or close mechanical contact, or by introduction into or application of suspensions of the phases to be integrated. Transportation of the phases to be integrated into the oxide coating to the substrate surface can be performed, or enhanced, by the application of electromagnetic fields.
Simultaneously or subsequently, in an electrochemical process step, the material forming the substrate surface is anodically polarized in an electrolyte solution.
On metallic materials that consist of valve metals or their alloys, this process step leads, through solution precipitation reactions, to an oxide growth at the phase boundary oxide coating/environment, followed by complete or partial integration of the phases at, or in the direct vicinity of, this phase boundary into the newly formed oxide coating.
The above process steps are, for the case of the integration of physiological organic components, carried out at or near room temperature so that both the structures and the functionality of these components is maintained.
The anodic polarization can be galvanostatically, potentiostatically or potentiodynamically performed until a predetermined formation potential has been reached. Criterion for the selection of the conditions of the anodic polarization is that the structure and functionality of the components to be integrated be optimally maintained in the process of the formation of the thin polyphase surface coatings.
The formation potential is selected to be in the range of 2 to 200 VSCE.
The advantage of the coatings produced according to the invention is that due to the firm integration of the organic and/or inorganic component into the oxide coating of the metallic material, an improved force transmission and permanent enhancement of the biocompatibility are achieved.
With the aid of the following examples the invention will be explained in more detail.